rustbuild: neither `./x.py doc --stage 0` nor `./x.py doc --stage 1 --keep-stage 0` regenerate api docs if only rustdoc changed

[QuietMisdreavus]

If i want to test a change to rustdoc by re-rendering the std docs, i could combine the "rebuild rustdoc" and "render api docs" steps into an ./x.py doc command. If i wanted to avoid rebuilding all of stage1, then i could try ./x.py doc --stage 0, but that doesn't actually re-render the documentation! It gets to that step, sees that the documentation output is newer than the source files for libstd, then skips it. (The same problem occurs with ./x.py doc --stage 1 --keep-stage 0, on a related note.) This requires a rustdoc contributor to rebuild an entire stage to be able to test their change on a rendering of std. (Testing it by rendering a separate crate is still possible, i suppose, but also requires the additional step of linking the toolchain in rustup or using the test rustdoc/rustc by a different means.)

[QuietMisdreavus]

This is kinda related to #38318, but not precisely. I'd made a comment in there explaining my findings when i first saw this behavior. However, looking back at that issue, that seems to be more about how --keep-stage isn't enabled by default, rather than not considering generated docs to need regeneration after a rustdoc change.